The present invention relates to gamma cameras and more particularly to direct conversion photon ray detectors.
Single photon emission computed tomography (SPECT) examinations are carried out by injecting a dilution marker comprising a compound labeled with a radiopharmaceutical into the body of a patient to be examined. A radiopharmaceutical is a substance that emits photons at energy levels within a known marker range. By choosing a compound that will accumulate in an organ of interest (i.e. an organ to be imaged), compound concentration, and hence radiopharmaceutical concentration, can be substantially limited to an organ of interest. A radiopharmaceutical that emits photons or gamma emissions which are approximately at a single known energy level is typically chosen.
While moving through a patient's blood stream the marker, including the radiopharmaceutical, becomes concentrated in the organ of interest. By measuring the number of photons emitted from the organ of interest which have an energy within the marker range, organ characteristics, including irregularities, can be identified.
To measure the number of emitted photons, one or more photon detecting cameras are used. After a marker has become concentrated within an organ of interest, a camera is positioned at an imaging angle with respect to the organ of interest such that the organ is positioned within the camera's field of view FOV. The camera is designed to detect photons traveling along preferred paths within the FOV.
Most gamma cameras consist of a collimator, a scintillation crystal, a plurality of photomultiplier tubes (PMTs) and a camera processor. For the purposes of this explanation cameras including a collimator, a crystal and PMTs will be referred to as scintillation cameras. The collimator typically includes a rectangular lead block having a width dimension and a length dimension which together define the FOV. The collimator block forms tiny holes which pass therethrough defining the preferred photon paths. The collimator blocks emissions toward the crystal along non-preferred paths.
The scintillation crystal is positioned adjacent the collimator on a side opposite the FOV and has an impact surface and an oppositely facing emitter surface. The impact surface defines a two dimensional imaging area. Photons which pass through the collimator impact and are absorbed by the impact surface at impact points. The crystal emitter surface emits light from an emitter point adjacent the impact point each time a photon is absorbed.
The PMTs are arranged in a two dimensional array which is positioned adjacent the emitter surface. Light emitted by the crystal is detected by the PMTs. Each PMT which detects light generates an analog intensity signal which is proportional to the amount of light detected. When a single photon is absorbed by the crystal, the emitted light is typically detected by several different PMTs such that several PMTs generate intensity signals simultaneously.
The processor receives all intensity signals and performs a plurality of calculations to determine precise coordinates X and Y on the impact surface at which a photon impact point occurred. Once coordinates X and Y of all photons have been identified, the processor uses coordinates X and Y to create an image of the organ of interest which corresponds to the camera imaging angle.
While scintillation cameras generate diagnostic quality images, these cameras have a number of shortcomings. First, because each scintillation camera includes a collimator, a crystal and a plurality of PMTs, each camera requires a relatively large volume. Also, while each of the collimator, crystal and PMTs separately is not extremely heavy, together the components are unwieldy. For these reasons it is difficult to manipulate scintillation cameras and most scintillation camera systems require a relatively complex and expensive supporting mechanism. For example, many scintillation cameras are mounted on large doughnut shaped gantries for rotation about an imaging axis.
In addition to increasing system costs, a gantry system capable of providing sufficient support to one or more scintillation cameras often renders the camera system immobile so that the system requires its own dedicated room and cannot be transported to an immobile patient (e.g. in an emergency room).
Furthermore, because each photon absorbed by the scintillation crystal causes a plurality of intensity signals (e.g. one intensity signal generated by each PMT), an extremely fast camera processor is required to process all of the intensity signals for identifying impact locations. In fact, in many cases processors are incapable of processing all intensity signals caused by each absorbed photon and therefore only a subset of intensity signals caused by each photon are processed, image accuracy sacrificed for speed.
To overcome the shortcomings associated with PMT cameras, the industry has developed a new generation of photon detectors generally referred to as direct conversion detectors DCDs. Most of these detectors are based on pixilated Cadmium Telluride CdTe or Cadmium Zinc Telluride CdZnTe devices. Generally, each DCD includes an absorption member, a cathode, at least one anode, a potential biasing mechanism (i.e. voltage source) and a separate amplifier for each anode.
The absorption member is formed of a planar semiconductor material (e.g. CdTe or CdZnTe) which has oppositely facing cathode and anode surfaces. The dimension between the cathode and anode surfaces is an absorption member thickness. When photons are directed at the cathode surface, the photons penetrate the absorption member and each photon is absorbed at an absorption depth within the member thickness. Photon absorption depths vary widely. When a photon interacts with the absorption member while being absorbed, the absorption member generates a plurality of electrons and holes.
The cathode is attached to and essentially covers the cathode surface and the anode is attached to the anode surface. The biasing mechanism is linked to the cathode and biases the cathode negative. The anode remains unbiased and therefore is positive with respect to the cathode. Because the cathode is negative and the anode is positive with respect to the cathode, when electrons and holes are generated during absorption, the holes are attracted to the cathode surface and the electrons are attracted to the anode surface. The electrons generate a first negative charge component on the anode.
As holes accumulate at the cathode, the positive charge adjacent the cathode causes a capacitive second negative charge component on the anode. To distinguish between the first negative charge component on the anode caused by electrons which travel from the absorption depth to the anode and the second negative charge component on the anode caused by the holes, the first negative charge component will be referred to hereinafter as the electron charge and the second negative charge component will be referred to hereinafter as the hole charge. Together, the electron charge and the hole charge are referred to hereinafter as the collected charge. The amplifiers do not distinguish between the electron charge and the hole charge and therefore amplify the entire collected charge.
The amplifier is attached to the anode and includes an output lead for providing an anode signal indicating the collected charge. The amplifier output lead is linked to a camera processor. The processor integrates the anode signal over an integration period and provides an intensity signal. The processor compares the intensity signal to an expected intensity signal associated with a photon having an energy within the marker range. When an intensity signal is equal to or exceeds the expected intensity signal, the processor indicates that a photon has been detected by the DCD which provided the anode signal.
Energy resolution (i.e. intensity signal resolution) of DCDs is primarily limited by three different phenomenon: (1) leakage current, (2) capacitive DCD noise associated with electronics used to configure the DCD and (2) incomplete charge collection.
The leakage current between the anode and cathode arises from two sources. First, there is a voltage difference between the anode and cathode and the material between them has a large but finite resistance. This leads to a small current flow between the anode and the cathode. Second, electron/hole pairs are thermally generated between the anode and cathode. These electron/hole pairs drift toward the anode and cathode, respectively, thus generating a small current. The amplifiers do not distinguish between the leakage current and the electron/hole charge caused by the absorbed photons and thus the leakage current operates as a noise source.
The leakage current can be reduced by several methods. First, by using highly resistive bulk material, the component of leakage current due to the finite resistance between the anode and cathode can be reduced. Second, by using bulk material which has a large gap between the conductive band and any populated (i.e. area having an electron) levels in the material, the number of thermally generated electron/hole pairs can be reduced. Third, the operating temperature of the DCD can be reduced, thus reducing the thermally generated leakage current. Fourth, by minimizing the integration time of the electronics, the amount of charge measured by integrating the leakage current is also minimized. The minimum integration time is determined by the time required for the signal from electrons and holes generated by a photon to be collected.
Most of the electronic noise is associated with the input capacitance of the amplifier (i.e. the DCD capacitance). Reducing the capacitance of the DCD therefore reduces this noise source.
In an effort to reduce DCD capacitance, the basic DCD configuration described above has been modified in several different ways. First, because DCD capacitance is inversely proportional to absorption member thickness, DCD capacitance can be reduced by increasing member thickness. In theory, DCD capacitance can be eliminated by increasing absorption member thickness. In reality, however, member thickness is limited by practical operational detector constraints.
Second, DCD capacitance is proportional to the areas of the cathode and anode. Therefore, by reducing the size of the DCD and providing a separate amplifier and processing circuitry for each DCD, total capacitance can be minimized.
Third, by reducing the size of the anode which collects the electron charge while essentially maintaining the size of the element on which the DCD capacitance forms, the overall effect of DCD capacitance can be substantially reduced. To this end, some members of the industry have developed detectors generally referred to as lateral drift detectors LDDs. A typical LDD includes an absorption member and a cathode as described above. However, an LDD includes at least one shaping electrode and one or more anodes, each of which is much smaller than the anode surface.
The anodes are spaced apart on and attached to the anode surface with electrode gaps therebetween. The electrode is attached to the anode surface between adjacent anodes and is configured such that a space exists between the electrode and each adjacent anode. The biasing mechanism biases each of the electrodes negative and also biases the cathode more negative than the electrode. Each anode is linked to a separate amplifier.
In operation, when a photon is absorbed and hole and electron pairs are generated, the shaping electrodes repel the electrons, all of which accumulate on one of the small anodes. The electrodes effectively add a transverse component to the electric fields. While essentially all of the electron charge accumulates on one of the anode, the capacitive charge is evenly distributed across the anode and electrode surface areas. Because the anode surface area comprises only a small portion of the combined areas of the anodes and electrode, capacitive noise is minimized.
In addition to DCD capacitance, incomplete charge collection adversely affects DCD energy resolution. Incomplete charge collection occurs when electrons and holes caused by an absorbed photon are not detected either (1) because of incomplete hole charge integration or (2) because of electron-hole recombination.
The time period required for electrons to traverse the distance from the absorption depth to the anode and for associated holes to traverse the distance from the absorption depth to the cathode will be referred to hereinafter as the collection period. Generally, it takes a longer time for a hole to travel to the cathode than it takes for an electron to travel to the anode. Therefore, the collection period is typically a measure of the time required for a hole to travel to the cathode and will depend on absorption depth. For example, a collection period corresponding to a first absorption depth will be shorter than a collection period corresponding to a second absorption depth which is relatively deeper and further from the cathode than the first depth.
Where the absorption depth is close to the cathode, the collection period will be relatively short. In this case, the processor's integration period will likely be longer than the collection period such that essentially all charge due to both holes and electrons is collected and integrated during the integration period. However, where the absorption depth is remote from the cathode and near the anode, the collection period may exceed the integration period such that the processor does not detect and integrate charge which is collected after the integration period (i.e. some hole charge due to slow traveling holes). In this case, the resulting intensity signal will underestimate the absorbed photon energy.
Incomplete charge collection also results from electron-hole recombination. As electrons and holes traverse toward the anode and cathode respectively, some holes and some electrons recombine within the absorption member and therefore never make it to the anode and cathode. Generally, because electrons travel through the absorption member at greater speeds than holes, more holes recombine than electrons. For this reason, the total charge detected by a processor also depends on the absorption depth. For example, if a photon is absorbed adjacent the cathode, most holes reach the cathode before recombining and a resulting intensity signal will essentially reflects the actual photon energy. However, if a photon is absorbed adjacent the anode so that holes have a longer distance to travel to the cathode, there is a good likelihood that many of the holes generated will be absorbed prior to reaching the cathode. In this case, the resulting intensity signal gain will indicate an incorrect energy level.
Several methods have been devised to compensate for, or minimize, incomplete charge collection in DCDs. According to a first method, because both the collection period and the extent of recombination are directly related to absorption depth, several detector systems have been designed which measure the rise time of the intensity signal and use the rise time to make a correction for incomplete charge collection. For example, on one hand, if intensity signal rise time is relatively slow, it can be assumed that the absorption depth is remote from the cathode and the intensity signal can be increased to compensate for incomplete charge. On the other hand, if intensity signal rise time is relatively quick, it can be assumed that the absorption depth is proximate the cathode and that the intensity signal reflects the actual energy level of the absorbed photon. In this case, the intensity signal is unchanged.
According to a second method, for a marker range, the fraction of gamma rays of a given energy that fall outside of the marker range due to incomplete charge collection can be measured during a quality control test for a given DCD. Multiple energy windows can then be used during imaging to map a portion of absorbed photons with energies below the marker range into the marker range.
According to a third method, by simply reducing anode sizes to a minimum and providing large shaping electrodes around the anodes, essentially all of the electron charge accumulates on a detecting anode and only a small fraction of the hole charge accumulates on the detecting anode. Therefore, only a small fraction of the hole charge error due to recombination or incomplete hole charge integration is detected by the detecting anode. A method of this type has been described in an article published in the 1995 IEEE Nuclear Science Symposium and Medical Imaging Conference Record, San Francisco, Cali., Vol. 1, pp. 544-548 entitled: "The Effect Of Pixel Geometry On Spatial And Spectral Resolution In A CdZnTe Imaging Array", by J D Eskin et al., which is incorporated herein by reference.
According to yet a fourth method, by interdigitating identically sized anodes and shaping electrodes where the anodes comprise precisely half of the total surface area of the anodes and electrodes combined, attaching the anodes and electrodes to the anode surface, connecting a first amplifier to the anodes, connecting a second amplifier to the shaping electrodes, anode and electrode signals, A and B, respectively, useable to identify a precise intensity signal can be generated. Anode signal A includes the entire electron charge. In addition, because the anodes comprise one half of the total surface area of the anodes and electrodes combined, anode signal A includes precisely one half of the hole charge while electrode signal B includes the other half of the hole charge. Subtracting electrode signal B from anode signal A then yields a modified signal which precisely indicates the electron charge. One example of this type of system is described in an article "Performance Of CdZnTe Coplanar-Grid Gamma-Ray Detectors, P. N. Luke and E. E. Eissler, IEEE Transactions on Nuclear Science, Vol. 43, No. 3, Jun. 1996, pp. 1481, which is incorporated herein by reference.
While each of the systems described above can generate photon imaging data, unfortunately the most accurate of the systems have a number of shortcomings. DCD detectors which are sufficiently accurate for diagnostic purposes use a large number of pixilated DCD elements, each element including a separate anode and corresponding low noise amplifier. The maximum size of the pixilated element is determined primarily by the spatial resolution required for imaging purposes. For example, to satisfy the sampling requirement for 4 mm spatial resolution, element dimensions should be smaller than 2 mm. Unfortunately, such small dimensions mean that a huge number of pixilated elements are required to configure a usefully sized camera head. For example, a camera having a FOV of 18 cm by 34 cm would contain approximately 15,300 2 mm by 2 mm elements, each element having a separate and dedicated amplifier and electronic channel. Such a large number of elements and electronic channels is undesirable for several reasons.
First, each element and corresponding channel must be separately configured. While the cost associated with each element and channel is not extremely high, camera configurations including several thousand channels associated with a DCD camera can be cost prohibitive
Second, each element and corresponding channel consumes approximately 1 milliwatt of power during operation to drive the amplifiers and channel electronics. Such high power consumption renders a fully configured DCD camera relatively inefficient and can require a complex cooling system to ensure that the DCD operates at a low temperature and therefore low leakage current.
Third, because each camera requires so many elements and channels, the probability of camera failure is quite high. For example, if the probability of failure per element and channel is 1 in one million, the probability of failure in the detector is approximately 1.5%. In most diagnostic environments a 1.5% probability of failure is unacceptably high.
Therefore, it would be advantageous to have a simple direct conversion detector configuration which requires a minimal number of amplifiers and electronic channels yet minimizes DCD capacitance, minimizes the effects of incomplete charge collection and increases both spatial and energy resolution.